Electromagnetic flow meter and magnetic excitation control method

Abstract

To suppress the generation of bubbles of gas dissolved in a fluid, a flow evaluating portion compares a measured flow rate value with a preset reference flow rate value and determines whether a fluid is flowing through a measurement tube. When the flow evaluating portion determines that the fluid is not flowing through the measurement tube, a magnetic excitation controlling portion imposes a limitation so that a magnetic excitation current reduces to an electric current value less than a magnetic excitation current obtained in normal measurement. When the flow evaluating portion determines that the fluid is flowing through the measurement tube, the magnetic excitation controlling portion releases the limitation on the magnetic excitation current.

The invention claimed is: 
     
       1. An electromagnetic flow meter, comprising:
 a magnetic excitation circuit configured to supply a predetermined magnetic excitation current to a magnetic excitation coil such that a magnetic field is applied to a fluid flowing through a measurement tube; 
 a pair of electrodes disposed facing each other on an inner wall of the measurement tube and configured to detect an electromotive force generated in the fluid by the magnetic field, wherein the electromagnetic flow meter is configured to calculate a measured flow rate value of the fluid based on the electromotive force; 
 a flow evaluating portion configured to compare the measured flow rate value with a present reference flow rate value to determine whether the fluid is flowing through the measurement tube; and 
 a magnetic excitation controlling portion configured to impose a limitation that reduces the predetermined magnetic excitation current to an electric current value less than the predetermined magnetic excitation current value when the flow evaluating portion determines that the fluid is not flowing, wherein the magnetic excitation controlling portion is configured to release the limitation on the predetermined magnetic excitation current when the flow evaluating portion determines that the fluid is flowing. 
 
     
     
       2. The electromagnetic flow meter according to  claim 1 , wherein
 the magnetic excitation controlling portion is configured to instruct the magnetic excitation circuit to switch the magnetic excitation current to the electric current value less than the predetermined electric current value when imposing the limitation on the magnetic excitation current and the magnetic excitation controlling portion is configured to instruct the magnetic excitation circuit to switch the magnetic excitation current to the predetermined electric current value when releasing the limitation on the magnetic excitation current. 
 
     
     
       3. An electromagnetic flow meter, comprising:
 a magnetic excitation circuit configured to supply a predetermined magnetic excitation current to a magnetic excitation coil such that a magnetic field is applied to a fluid flowing through a measurement tube; 
 a pair of electrodes disposed facing each other on an inner wall of the measurement tube and configured to detect an electromotive force generated in the fluid by the magnetic field, wherein the electromagnetic flow meter is configured to calculate a measured flow rate value of the fluid based on the electromotive force; 
 a comparator configured to perform alternate current amplification of the electromotive force, the comparator configured to compare a direct current flow rate voltage obtained by sampling and holding with a preset reference flow rate voltage, and determining whether the fluid is flowing through the measurement tube, wherein 
 the magnetic excitation circuit is configured to impose a limitation to reduce the magnetic excitation current to an electric current value less than the predetermined magnetic excitation current when the comparator determines that the fluid is not flowing and the magnetic excitation circuit is configured to release the limitation on the magnetic excitation current when the comparator determines that the fluid is flowing. 
 
     
     
       4. The electromagnetic flow meter according to  claim 3 , wherein
 the magnetic excitation circuit is configured to switch the magnetic excitation current to the electric current value less than the predetermined electric current value when imposing the limitation on the magnetic excitation current and the magnetic excitation circuit is configured to switch the magnetic excitation current to the predetermined electric current value when releasing the limitation on the magnetic excitation current. 
 
     
     
       5. A magnetic excitation control method for an electromagnetic flow meter, comprising:
 supplying a predetermined magnetic excitation current from a magnetic excitation circuit to a magnetic excitation coil so that a magnetic field is applied to a fluid flowing through a measurement tube; 
 detecting an electromotive force generated in the fluid according to the magnetic field using a pair of electrodes disposed so as to face each other on an inner wall of the measurement tube; 
 calculating a measured flow rate value of the fluid based on the electromotive force; 
 in a flow determination step, comparing the measured flow rate value with a present reference flow rate value and determining whether the fluid is flowing through the measurement tube; and 
 in a magnetic excitation control step, imposing a limitation so that the magnetic excitation current reduces to an electric current value less than the predetermined magnetic excitation current when the flow determination step determines that the fluid is not flowing and releasing the limitation on the magnetic excitation current when the flow determination step determines that the fluid is flowing. 
 
     
     
       6. The magnetic excitation control method according to  claim 5 , wherein
 the flow determination step performs alternate current amplification of the electromotive force, compares a direct current flow rate voltage obtained by sampling and holding with a preset reference flow rate voltage, and determines whether the fluid is flowing through the measurement tube.